1. Field of the Invention
The present invention relates to a heat treatment jig that is used in a heat treatment boat of a vertical heat treatment furnace and a heat treatment method, and in more detail, to a heat treatment jig for a semiconductor substrate that can suppress slip generated as a crystal defect when the semiconductor substrate is heat treated at a high temperature from occurring and a heat treatment method therewith.
2. Description of the Related Art
A semiconductor substrate that is processed in an LSI device fabrication process, in steps of oxidation, diffusion and film deposition, is repeatedly subjected to high temperature heat treatment to fabricate. At the time of the heat treatment, when an inhomogeneous temperature distribution is caused within a plane of the semiconductor substrate, in accordance therewith the thermal stress is generated in the semiconductor substrate.
On the other hand, depending on a supporting method of the semiconductor substrate, owing to a weight of a wafer (substrate), the stress (hereinafter referred to simply as “tare stress”) is generated. In an existing heat treatment boat, in the case of the silicon substrate being supported only by a periphery portion thereof, when a semiconductor substrate having a diameter of 300 mm is heat treated, the tare stress is largely generated, resulting in difficulty to use.
It is known that the thermal stress and the tare stress generate, in the heat treatment, crystal defect called the slip in the semiconductor substrate. The slip causes an increase in the leakage current of the LSI device and deteriorates the planarity of the semiconductor substrate. In order to secure the quality characteristics of the semiconductor substrate, in order to suppress the slip from occurring, it is important to reduce the thermal stress and the tare stress.
In heat treating the semiconductor substrate, a vertical heat treatment furnace can reduce an installation space and is suitable for heat treating a lot of semiconductor substrates having a large diameter; accordingly, it is adopted as apparatus that is used in various heat treatments of semiconductor substrates.
FIG. 1 is a diagram showing a configuration example of a heat treatment boat for use in semiconductor substrates that is used in the vertical heat treatment furnace. A heat treatment boat 1 includes three or more pillars 3 and a top panel 5 and a bottom panel 6 that fix these pillars 3 up and down positions, and is provided with an opening 2. The pillar 3 is provided with substrate supporting portions 4 arranged in parallel. After the semiconductor substrates are placed from a side of the opening 2 on the substrate supporting portions 4, the boat is inserted into the vertical heat treatment furnace followed by carrying out predetermined heat treatment.
As shown in FIG. 1, the heat treatment boat 1 is constituted of a pair of top and bottom panels 5, 6 disposed distanced with a separation and a plurality of pillars 3 that links the top and bottom panels; accordingly, the opening 2 is necessarily disposed to place the semiconductor substrates on the substrate supporting portions 4 or to take out therefrom. Accordingly, normally, two pillars 3 disposed on a side of the opening 2, in order to easily place or take out the semiconductor substrates, are disposed distanced with a separation substantially equal to a diameter of the semiconductor substrate.
In heat treating small diameter semiconductor substrates, a method has been widely used in which by use of the heat treatment boat 1 shown in FIG. 1, an outer periphery portion of a back surface of the substrate is supported at a plurality of points of substantially three or four points. However, with a recent increase in a diameter of the semiconductor substrate, largely increased tare stress became to be generated. As a result, in the method in which the outer periphery portion of a back surface of the substrate is supported at three or four points by means of the heat treatment boat 1 shown in FIG. 1, in accordance with an increase in the tare stress at the supporting points, the inhomogeneous distribution of the tare stress is forwarded, resulting in abundant generation of the slips.
Accordingly, recently, in order to reduce the occurrence of the slips in conjunction with an increase in the diameter of the semiconductor substrate, a jig that supports the inside of the back surface of the semiconductor substrate at a plurality of points, or a jig that supports the semiconductor substrate by a ring-like line contact or surface contact with the semiconductor substrate has been put into practical use.
By use of the jig that is put into practical use and supports the inside of the back surface of the semiconductor substrate, the slips that are caused when the semiconductor substrate is heat treated can be reduced. However, in order to reduce the slips with precision by use of these heat treatment jigs that have a structure in which contact method is altered, problems are newly caused.
For instance, since the back surface of the semiconductor substrate adheres to the supporting jig, the semiconductor substrate and the supporting jig each are restricted in the deformation. Accordingly, in some cases, large stress that exceeds the thermal stress or the tare stress may be added anew to the semiconductor substrate. In such a case, newly added stress generates the slip.
As a new cause of the slip generation, there is one due to the processing accuracy of the heat treatment jig itself; in particular, the slip generation largely depends on the flatness and the surface roughness of surfaces in a region where the back surface of the semiconductor substrate and the supporting jig come into contact.
Ordinarily, the heat treatment jig that is used in high temperature heat treatment is manufactured of silicon carbide. Accordingly, the flatness in a region where the semiconductor substrate and the supporting jig come into contact is 200 μm or less, and a jig surface has a structure where surface irregularities are locally put together.
In this connection, the present inventors have proposed a heat treatment method in which a silicon carbide holder having a thickness of less than 1 mm is disposed, thereon a silicon or silicon carbide ring or disc jig smaller in a diameter than the semiconductor substrate is disposed, and further thereon the semiconductor substrate is placed to perform the heat treatment (Japanese Patent Application Laid-open No. 2001-358086).
Similarly, the present inventors disclose a heat treatment jig in which on a silicon carbide heat treatment jig that is used in a vapor-phase growth method, a center protrusion that supports a center portion of a back surface of a semiconductor substrate and at least one circular arc that supports the back surface of the substrate are disposed, and on all or part of a contact surface of the center protrusion and the circular arc that come into contact with the semiconductor substrate a capping member is disposed (Japanese Patent Application Laid-open No. 2003-037112).
On the other hand, Japanese Patent Application Laid-open Nos. 9-199438 and 10-270369 due to other inventors than the present inventors disclose a method in which by use of a supporting jig formed into a disc-like or ring-like shape, silicon substrates are placed on these supporting jigs followed by applying heat treatment. However, these publications do not describe of the flatness of the disc-like or ring-like supporting jig and the aggregate of the concave and salient.
According to the heat treatment method proposed in Japanese Patent Application Laid-open No. 2001-358086, under very high temperature heat treatment conditions that exceed 1300° C. in the heat treatment temperature and are longer than 10 hours in the holding time period such as the heat treatment of SIMOX substrate, silicon carbide itself that constitutes the holder becomes weaker in the mechanical strength. Accordingly, since a thickness of the holder is less than 1 mm, the bending cannot be suppressed from occurring; as a result, also in the ring or disc jig that the holder supports, similarly the bending is generated. Finally, the slips are generated even in the semiconductor substrate.
As mentioned above, the reason for the slips being generated in the semiconductor substrate is due to the lack of the mechanical strength of the silicon carbide holder and furthermore the ring or disc jig that the holder supports under very high temperature heat treatment conditions. Furthermore, depending on the conditions of contact with the semiconductor substrate, the adherence at a contact surface between a small diameter disc or ring jig and the semiconductor substrate becomes stronger; accordingly, the slips due to local adherence also are generated.
Even when with the jig disclosed in Japanese Patent Application Laid-open No. 2003-037112, heights of the circular arc and the center protrusion on the holder are processed with precision, owing to the bending of the holder generated during the high temperature heat treatment, an error is generated between the heights of the circular arc and the center protrusion on the holder, resulting in difficulty in diminishing the slips.
In order to diminish the error generated during the high temperature heat treatment, a thick holder can be ground out of sintered silicon carbide, and therewith a center protrusion and a circular arc that are difficult to exhibit the bending can be fabricated. However, since manufacturing cost increases, this cannot be applied to industrial mass-production.
Furthermore, even when the disc-shaped or the ring-shaped supporting jig described in Japanese Patent Application Laid-open Nos. 9-199438 and 10-270369 is used, under the very high temperature heat treatment conditions, the slips occur in the semiconductor substrate. Thus, the use of the disc-shaped or ring-shaped supporting substrate alone cannot suppress the slips that are generated in conjunction with the heat treatment from occurring.